This project extends our long-standing interests in studying human infectious diseases and therapy in relevant in vivo models. HIV (human immunodeficiency virus) and HCV (hepatitis C virus) dual infection is highly prevalent as a result of shared transmission routes. In a number of recent epidemiological studies, HIV-1 co-infection has been correlated with accelerated liver disease progression in HCV/HBV infected patients. A small animal model for studying HCV/HIV immuno-pathogenesis is urgently needed. All current mouse models allow only HCV replication in the absence of a functional immune system. Those models are not useful to study host immune responses, their contributions to HCV pathogenesis and potential immunotherapeutic approaches. The Rag2-gammaC double knockout (DKO) mouse lacks T/B and NK cells, and allows development of a functional human immune system with human HSC (DKO-hu HSC). Normal human T, B, and dendritic cells are present in lymphoid tissues such as thymus, spleen, peripheral blood (PB), liver and lymph nodes (LN). We also show that DKO-hu-HSC mice support HIV-1 infection with high viremia in the PB/plasma. Human CD4+ T cells are gradually depleted by HIV-1 in a dose-dependent manner. In addition, HIV-1 infection persisted in infected HSC-DKO mice for >20 weeks, with infectious HIV-1 in lymphoid tissues. Furthermore, HIV-1 or EBV infection induces human immune responses. Lastly, human hepatocytes are also co-engrafted with human immune system in the DKO mouse co-transplanted with human fetal liver derived hepatocyte progenitor cells with CD34+ HSC cells (DKO-hu HSC/Hep mouse). The long-term goals of this project are to establish a relevant mouse model to study the infection and immuno- pathogenesis of HCV, and to test novel immune-based therapeutics to overcome immune tolerance, inflammation, fibrosis and HCC. We will first establish the DKO-hu HSC/Hep model for studying HCV infection and pathogenesis. We will investigate HCV replication and pathogenesis in the absence or presence of HIV-1 co-infection in vivo in the model. I also propose to increase human hepatocyte engraftment in the DKO-hu HSC/Hep mouse by selectively depleting murine hepatocytes and promoting human hepatocytes in the DKO- hu HSC/Hep mouse. These studies will establish the foundation for future study and shed light on novel therapeutic strategies for controlling HCV diseases. This model will also be useful to test novel anti-HCV (HCC) therapeutics. Project Narrative: This project will establish a novel animal model for studying human hepatocyte development and HCV infection and pathogenesis. We will investigate HCV replication and pathogenesis in the absence or presence of an intact human immune system in vivo in the model. These studies will establish the foundation for future study and shed light on novel therapeutic strategies for controlling HCV diseases.